Ferrite thin film for high frequency and method for preparation thereof

ABSTRACT

The present invention provides a Y-type hexagonal ferrite thin film suitable for high frequency devices, having a crystal structure with the c-axis oriented perpendicular to the surface of the thin film. The present invention also provides a method of producing the Y-type hexagonal ferrite thin film, comprising the steps of preparing a viscous solution containing a metal-organic complex which is formed using a primary component including a Fe +3  ion, and a secondary component including a Ba 2+  ion, at least one transition metal ion selected from the group consisting of Fe 2+ , Co 2+ , Ni 2+ , Zn 2+ , CU 2+  and Mn 2+ ; and optionally at least one metal ion selected from the group consisting of Sr 2+ , Ca 2+  and Pb 2+ , forming a film having a Y-type ferrite composition on a surface made of noble metal through a coating process using the viscous solution, and burning the film at a temperature of  750 ° C. or more.

TECHNICAL FIELD

[0001] The present invention relates to a ferroxplana-type hexagonalferrite thin film capable of obtaining a high magnetic permeability inthe high frequency range of high-frequency communication devices or thelike, and a production method thereof.

BACKGROUND ART

[0002] Heretofore, cubic spinel-type ferrites as represented by Mn—Znferrite have been used in high frequency devices by taking advantage oftheir high magnetic permeability. However, upon use in frequencies ofseveral hundred MHz, the permeability of the cubic spinel-type ferritesis sharply deteriorated due to their Snoek's limit, and theeffectiveness as material for high-frequency devices will disappear.Among hexagonal ferrites, a ferroxplana-type ferrite with the c-planehaving a high magnetizability is expected as noteworthy material forhigh frequency devices to be used in higher frequency range, because itcan maintain a high magnetic permeability up to several GHz beyond theSnoek's limit of cubic spinel-type ferrites. The ferroxplana-typeferrite has a typical composition of Ba₂Zn₂Fe₁₂O₂₂ or Ba₃Co₂Fe₂₄O₄₁.There have also been known more complicated compositions such as acomposition including SiO₂ and CaO in addition to the above composition(Japanese Patent Laid-Open Publication No. H09-129433) or Ba₃Co₂ (M_(x),N_(x)) Fe_(24-2x)O₄₁ (M: divalent metal ion such as Zn, Cu or Co; N:quadrivalent metal ion such as Ti, Zr, Hf, Si, Ge, Sn or Ir; x: 3 orless; Japanese Patent Laid-Open Publication No. 2000-235916). Thesehexagonal ferrites have been used as powder material for a sintered body(Japanese Patent Laid-Open Publication No. H09-129433) or powder pastefor a coated layer (Japanese Patent Laid-Open Publication No.H09-205031).

[0003] Recently, in connection with advance of information andcommunications apparatuses such as portable phones and personalcomputers, downsizing and increase in signal frequency of electronicdevices have been accelerated, which leads to the need for developinghigh-frequency electronic devices such as a filter or inductor availablein higher frequency range with more downsized structure. As a recenttrend in downsizing, in view of the limit of a traditional approach of3-dimensionally downsizing a bulk device, a planer device effective todownsizing and integration is actively developed by utilizing atechnology of laminating thin films. However, despite of the strong needin small devices, no technology of forming a thin film using aferroxplana-type ferrite has been successfully developed.

[0004] As shown in FIG. 1, a hexagonal ferrite has M-type, U-type,W-type, X-type, Y-type and Z-type phases, and these phases havedifferent solid-solution ranges, respectively. In addition, the crystalstructure in each of the phases is extremely complicated as illustratedin FIG. 2. Thus, while there have been reported many cases of theformation of M-type (BaFe₁₂O₁₉) thin films which is binary system andhas perpendicular magnetic anisotropy, none of the formation of othertype hexagonal ferrite thin films has been reported. The M-typehexagonal ferrite is a magnetoplumbite-type ferrite having uniaxialanisotropy, and is thereby used for quite different purposes from thoseof other type hexagonal ferrites. Therefore, the need for developing atechnology of forming a Y-type hexagonal ferrite thin film usable inhigh frequency devices strongly exists.

SUMMARY OF THE INVENTION

[0005] As a result of various researches for solving the above problem,the inventors has found that a thin film of Y-type hexagonal ferritebelonging to ferroxplana-type ferrites can be produced through a methodof forming a film on a noble metal surface by use of a viscous solutioncontaining a metal-organic complex.

[0006] Specifically, the present invention provides a Y-type hexagonalferrite thin film having a crystal structure with the c-axis orientedperpendicular to the surface of the thin film.

[0007] The present invention also provides a method of producing theabove Y-type hexagonal ferrite thin film. This method comprises thesteps of preparing a viscous solution containing a metal-organic complexwhich is formed using a primary component including a Fe⁺³ ion, and asecondary component including a Ba²⁺ ion and at least one transitionmetal ion selected from the group consisting of Fe²⁺, Co²⁺, Ni²⁺, Zn²⁺,Cu²⁺ and Mn²⁺, forming a film having a Y-type ferrite composition on asurface made of noble metal, by use of the viscous solution, and burningthe film. The secondary component may further include at least one metalion selected from the group consisting of Sr²⁺, Ca²⁺ and Pb²⁺.

[0008] In this method, the preparing step may include the step of addingorganic acid and polyol to a solution of material prepared usingwater-soluble compounds containing Ba, Zn and Fe as starting materialsto form the metal-organic complex constituting the viscous solution,wherein the viscous solution is formed as a film having a composition ofY-type Ba₂Zn₂Fe₁₂O₂₂ on the surface made of noble metal and then burnt.

[0009] In the method, the burning step may be performed at a temperatureof 750° C. or more to obtain the above Y-type hexagonal ferrite thinfilm.

[0010] In the present invention, the composition of the Y-type hexagonalferrite thin film having a crystal structure with the c-axis orientedperpendicular to the surface of the thin film is typically expressed bya general formula Ba₂Me₂Fe₁₂O₂₂ (Me: at least one transition metalselected from the group consisting of Ni, Co, Zn, Cu and Mn).Alternatively, the Y-type hexagonal ferrite thin film may have acomposition formed by substituting Ba in the above basic compositionwith at least one metal selected from the group consisting of Sr, Ca andPb, or a more complicated Y-type hexagonal Ba ferrite composition formedby adding a small amount of various elements, such as B, Si or Mg, atany position in the above basic composition.

[0011] Heretofore, a solid-phase bulk Y-type ferrite has been burnt atabout 1000° C. By contrast, in the method of the present invention, afilm having a Y-type ferrite composition is formed on a surface made ofnoble metal to allow a Y-type hexagonal ferrite thin film having acrystal structure with the c-axis oriented perpendicular to the surfaceof the thin film to be obtained at a lower burning temperature. If thesurface is made of Al₂O₃, the reaction between the ferrite and Al₂O₃will preclude any thin film formation. If the surface is made of quartz(SiO₂), no Y-type hexagonal ferrite thin film can be obtained because aformed film will be solid-solved in SiO₂.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a phase diagram of hexagonal ferrites.

[0013]FIG. 2 is a schematic diagram showing the atomic arrangement of aY-type hexagonal Ba ferrite.

[0014]FIG. 3 illustrates a process of producing a tiny planar deviceusing a Y-type hexagonal ferrite thin film having a crystal structurewith the c-axis oriented perpendicular to the surface of the thin film,wherein FIG. 3(A) is a top plan view, and FIG. 3(B) is a side view.

[0015]FIG. 4 is a graph showing the XRD pattern of a thin film producedin EXAMPLE 1.

[0016]FIG. 5 is a graph showing the magnetization curve of the thin filmproduced in EXAMPLE 1.

[0017]FIG. 6 is a photographic representation of the SEM image of thethin film produced in EXAMPLE 1.

[0018]FIG. 7 is a graph showing the XRD pattern of a thin film producedin EXAMPLE 2.

[0019]FIG. 8 is a graph showing the magnetization curve of the thin filmproduced in EXAMPLE 2.

[0020]FIG. 9 is a graph showing the XRD pattern on an M-type-Y-type lineof a thin film produced in COMPARATIVE EXAMPLE 1.

[0021]FIG. 10 is a phase diagram showing the composition of the thinfilm produced in COMPARATIVE EXAMPLE 1.

[0022]FIG. 11 is a graph showing the XRD pattern on a BaFe₂O₄—Y-typeline of a thin film produced in COMPARATIVE EXAMPLE 2.

[0023]FIG. 12 is a graph showing the Moessbauer spectrum on aBaFe₂O₄—Y-type line of the thin film produced in COMPARATIVE EXAMPLE 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0024] A method of producing a Y-type hexagonal ferrite thin film havinga crystal structure with the c-axis oriented perpendicular to thesurface of the thin film, according to the present invention, will nowbe described in detail.

[0025] Firstly, a viscous solution containing a metal-organic complexformed using a stating material capable of supplying a Fe⁺³ ion as aprimary component, another kind of divalent transition metal ion, a Ba²⁺ion, and optionally a Sr²⁺, Ca²⁺ or Pb²⁺ ion is prepared. The term“viscous solution” means a glutinous or sticky liquid, and morespecifically an aqueous fluid in the form that a polymeric metal-organiccomplex is transparently and homogeneously dispersed in a solution suchas water, acetic acid or ethanol.

[0026] This viscous solution may be prepared through (1) a process ofslowly concentrating an initial water solution added with the startingmaterial while gradually vaporizing the water in the solution until ithas a desired viscosity, or (2) a process of vaporizing the entire waterin an initial water solution added with the starting material to obtaina gel metal-organic complex, and then dissolving the gel metal-organiccomplex in a solvent such as water, acetic acid, or ethanol to provide asolution having a desired viscosity.

[0027] As a specific example, a so-called metal complex polymerizationprocess may be preferably used to prepare the viscosity solution. In thecomplex polymerization process, organic acid, such as citric acid, anddiol, such as ethylene glycol, propylene glycol or butane diol, or anyother suitable polyol, are added to a water solution which contains astarting material, such as metal carbonate, metal hydroxide, metalsulfate, metal carboxylate or metal halogenide, dissolved therein toprovide a given ferrite composition, and then the obtained mixture isdehydrated and condensed to crosslink the metal ions with each otherthrough the organic matters so as to form an metal-organic complex.

[0028] The metal complex polymerization process itself is a knowntechnology for homogeneously dispersing metal ions throughcomplexifization of the metal ions using citric acid and formation of a3-dimensional network using ethylene glycol. More specifically, a stablechelate complex is first formed between citric acid and plural kinds ofmetal ions. Then, the chelate complex is dispersedly dissolved inethylene glycol, and the obtained solution is copolymerized andesterified under heating to form an oligomer and finally form a polymergel having a 3-dimensional network structure or a complexpolymerization. The complex polymerization process is used inmanufacturing a complex oxide such as superconducting material.

[0029] The obtained polymer gel as a precursor has a significantlystable network structure primarily formed through ester bonding orcopolymerization. Thus, the mobility of the metal ions is drasticallyreduced to allow ceramic particles to be formed directly therefrom witha desired composition in a subsequent process.

[0030] As compared to a sol-gel process using alcoxide as material, themetal complex polymerization process has advantages of enhancedhomogeneity in metal-ion dispersion, and lower cost in material.Further, when the metal complex polymerization process is used toproduce a multinary thin film, the enhanced homogeneity in dispersion ofmetal-ion can be advantageously maintained until a burning step byvirtue of the 3-dimensional network of the polymer. Any suitableconventional coating or printing process, such as a dip coating or spincoating process, may be used to form a film. Preferably, in each processof applying the polymer gel, the thickness of a film to be applied isset in the range of about 80 to 100 nm. If it is required to increasethe film thickness, the processes of film formation, drying,polymerization and thermal decomposition may be repeated.

[0031] In the dip coating process, after immersion into a coatingliquid, a substrate is taken out of the coating liquid upward, andsubjected to a treatment for gelatinizing the liquid film deposited onthe surface of the substrate. The dip coating process has variousadvantages of no need for expensive large-scale facility as in achemical vapor deposition process, availability for any size of asubstrate, simple operation, and applicability in producing a multinarythin film.

[0032] As a pretreatment of the surface made of Ag, Au, Pt or othermetal of the platinum family, on which a film having a composition ofY-type ferrite is formed, it is desired to rinse the surface of an oxidelayer formed on the surface of the noble metal naturally or due tothermal oxidation, with a basic water solution such as KOH to modify thesurface of the oxide layer completely by an OH group. Alternatively, thesurface of the oxide layer may be simply rinsed with distilled water orhydrophilic liquid such as ethanol to obtain substantially the sameeffect. In case of Ag, even if its surface is briefly rinsed, it allowsa ferrite film to adequately deposited thereonto because an Ag oxidelayer is naturally formed thereon in the atmosphere. In case of Au orPt, almost no oxide layer is naturally formed on its surface. Thus, itis preferable to heat the surface under oxygen atmosphere or irradiatethe surface with oxygen plasma so as to positively form an oxide layeron the surface to provide sufficient wettability of a viscous solutionduring coating process or enhanced deposition of a film. The Ag, Au, Ptor other metal of the platinum family may contain any alloy element aslong as an intended function of its surface is not lost.

[0033] While the above noble metal has a cubic close-packed crystalstructure, its surface can have an elongated directional microstructuredue to rolling or scratch. In this case, the crystal orientation in aY-type hexagonal ferrite thin film can be enhanced by coating a film ina direction perpendicular to the direction of the microstructure ratherthan coating it in parallel thereto.

[0034] The conventional bulk device has a limit in 3-dimensionalreduction in size, e.g. minimum thickness of about 3 mm or minimumradius of 2.5 mm, due to its structure composed of a plurality ofseparated components. By contrast, a Y-type hexagonal ferrite thin filmof the present invention can be used to provide a device having adesired shape by coating a Y-type ferrite film on a surface made ofnoble metal while placing on the surface a mask with an openingcorresponding to the desired shape, or by forming the Y-type hexagonalferrite thin film over the surface and partially removing the thin filmwith etching to allow the remaining thin film to have the desired shape.

[0035] For example, as shown in FIG. 3, a significantly downsized planardevice having a thickness of about 20 μm and a radius of about 1 mm byforming a lower coil (a) having a surface layer made of one noble metalselected from the group consisting of Ag, Au, Pt and other metal ofnoble metal family, on a substrate 1 made of insulating orsemiconducting material such as Al₂O₃ or Si, forming the Y-typehexagonal ferrite thin film (b) of the present invention on the surfacelayer, etching the thin film (b) to leave a ring-shaped thin film, andforming an upper coil (c) on the ring-shaped thin film.

[0036] In the method of the present invention, when the film having theY-type ferrite composition is burnt after formed on the surface made ofnoble metal, the crystal orientation in a Y-type hexagonal ferrite thinfilm to be obtained is deteriorated as the burning temperature isreduced. Thus, in either noble metal constituting the surface, theburning temperature should be set at 750° C. or more to assure anenhanced in-plane crystal orientation in the obtained thin film. Theburning temperature of less than 750° C. precludes the formation of theintended Y-type hexagonal ferrite thin film having a crystal structurewith the c-axis oriented perpendicular to the surface of the thin film,and the crystal orientation will be enhanced as the burning temperatureis increased.

[0037] The upper limit of the burning temperature is preferably set at900° C. for the surface made of Ag (melting point: 960.8° C.), or at1300° C. for the surface made of Pt (melting point: 1769° C.), or at1000° C. for the surface made of Au (melting point: 1063° C.). In thesurface made of Ag or Au, upon exceeding the upper limit, the burningtemperature gets close to their melting point, and the smoothness of theobtained thin film is sharply deteriorated. In the surface made of Pt,at a temperature exceeding the upper limit, the ferrite itself in thecoated film is partially molten and decomposed. It is particularlypreferable to use Ag as the material of the surface, because the burningtemperature for the surface made of Ag is set less than the meltingpoint (962° C.) of a silver wire for use as electrical leads to allowsilver wire to be burned together with the coated film. In view of thesituation where high frequency devices generally employ a silver wire intheir circuit, the Y-type hexagonal Ba ferrite thin film formed on thesurface made of Ag is a significantly effective material for highfrequency devices.

EXAMPLE Example 1

[0038] Starting materials, BaCO₃, ZnCO₃ and FeCl₃.6H₂O were weighted tohave their ratio of 1:1:6, and put in a beaker. On the basis of thetotal mol number of the starting materials, 70-time mol of distilledwater, 3.75-time mol of citric acid, and 11.25-time mol of ethyleneglycol were added to the starting materials to form a metal complex.

[0039] The obtained viscous solution was heated at 90° C. by a hotstirrer, and concentrated. After the stirrer is completely stopped, theobtained gel was diluted by adding 2 parts of acetic acid on the basisof the weight of the gel. A pure Ag substrate (0.20×10×10 mm, availablefrom Nilaco Co.) is dipped in the viscous solution prepared in the aboveway, and taken out of the viscous solution upward at a constant speed toform a film coated on the surface of the Ag substrate. The coated filmwas subjected sequentially to drying at 90° C. for 1 hour,polymerization at 190° C. for 1 hour, and thermal decomposition at 410°C. for 1 hour. In the above process, when the substrate was taken out ofthe viscous solution upward at a constant speed of 43.7 mm/min, a coatedfilm had a thickness of about 1000 Å. Then, the substrate with thecoated film was burnt at various temperatures.

[0040] The obtained thin film was evaluated by identifying the createdphase using XRD, measuring the magnetization using VSM, observing thesurface of the thin film using SEM, determining the magnetization phasethrough Moessbauer spectroscopy, and measuring the thickness of the thinfilm. The XRD pattern of the produced thin film is shown in FIG. 4. AY-type peak appeared in the thin film burnt at 900° C. for 1 hour. Inaddition, despite of Ag having a cubic crystal structure, all of theobtained Y-type peaks were oriented in the direction of (001) plane.

[0041] The magnetization curves of the above thin film are shown in FIG.5. As compared to the hysteresis curve obtained by applying a magneticfield in the vertical direction of the thin film, the hysteresis curveobtained by applying a magnetic field in the in-plane direction of thethin film has a sharper rising, and consequently a smaller coercivity.That is, the produced thin film has high in-plane magnetizability. TheSEM image of the thin film is shown in FIG. 6. Hexagonal plate-shapedgrains observed in FIG. 6 prove the creation of a hexagonal crystalstructure.

Example 2

[0042] A thin film was produced under the same conditions as those inEXAMPLE 1 except for using a pure Pt substrate (0.20×10×10 mm, availablefrom Nilaco Co.) and coating a film on the surface of the Pt substrate.The XRD pattern of the produced thin film is shown in FIG. 7. When asurface made of Pt was used, a Y-type peak appeared in the produced thinfilm burnt at 1100° C. for 1 hour. In addition, despite of Pt having acubic crystal structure, all of the obtained Y-type peaks were orientedin the direction of (001) plane.

[0043] The magnetization curves of the above thin film are shown in FIG.8. As compared to the hysteresis curve obtained by applying a magneticfield in the vertical direction of the thin film, the hysteresis curveobtained by applying a magnetic field in the in-plane direction of thethin film has a sharper rising, and consequently a smaller coercivity.That is, the produced thin film has high in-plane magnetizability.

Comparative Example 1

[0044] A thin film was produced under the same conditions as those inEXAMPLE 1 except for using a α-Al₂O₃ c-plane substrate and coating afilm on the surface of the Al₂O₃ substrate. A coated film was burnt at1100° C. for 30 minutes. The XRD pattern on an M-type-Y-type line of theproduced thin film is shown in FIG. 9, and the composition of theproduced thin film is plotted on a phase diagram in FIG. 10. Whenα-Al₂O₃ c-plane was used as a surface, only peaks of the thin filmreacted with the substrate appeared. While an M-type Ba ferrite phasecould be observed in all of the compositions of the produced thin filmas shown in FIG. 9, no Y-type phase could be created.

Comparative Example 2

[0045] In COMPARATIVE EXAMPLE 1, there is a possibility that Zn having alow melting point was released outside the phases. Thus, a coated filmunder the same conditions as those in EXAMPLE 1 was burnt at a lowertemperature of 1000° C. In this case, a previously unknown phaseappeared. In view of the peak of this unknown phase exhibited at aposition close to a hexagonal α-Al₂O₃ substrate and the burningtemperature, it can be assumed that the unknown phase is also hexagonalα′-BaFe₂O₄. The XRD pattern on a BaFe₂O₄—Y-type line of the producedthin film is shown in FIG. 11. As seen in FIG. 11, an unknown phaseappeared in the composition having Zn. FIG. 12 is the Moessbauerspectrum on a BaFe₂O₄—Y-type line of the produced thin film. A peak inthe composition of BaFe₂O₄ shows only an antiferromagnetic phase, whichresults from α′-BaFe₂O₄.

[0046] However, the peak of the antiferromagnetic phase goes down andthe peak of a paramagnetic phase goes up as the composition gets closeto Y-type. It is believed that the paramagnetic phase is a compound(AlFeO₃) similar to an ilmenite structure created through the reactionbetween Fe in the components of the coated film and Al in the componentsof the substrate. Through this example, it was proved that the intendedthin film cannot be obtained using the α-Al₂O₃ substrate due to thereaction between the coated film and the substrate.

Industrial Applicability

[0047] The Y-type hexagonal ferrite thin film of the present inventionis useful as material for high-frequency electronic devices such as afilter or inductor available in higher frequency range.

What is claimed is:
 1. A Y-type hexagonal ferrite thin film having acrystal structure with the c-axis oriented perpendicular to the surfaceof said thin film.
 2. A Y-type hexagonal ferrite thin film for use in ahigh frequency device, having a crystal structure with the c-axisoriented perpendicular to the surface of said thin film.
 3. A method ofproducing the Y-type hexagonal ferrite thin film as defined in claim 1or 2, comprising the steps of: preparing a viscous solution containing ametal-organic complex which is formed using a primary componentincluding a Fe⁺³ ion, and a secondary component including a Ba²⁺ ion andat least one transition metal ion selected from the group consisting ofFe²⁺, Co²⁺, Ni²⁺, Zn²⁺, Cu²⁺ and Mn²⁺; forming a film having a Y-typeferrite composition on a surface made of noble metal, by use of saidviscous solution; and burning said film.
 4. The method as defined inclaim 3, wherein said secondary component further includes at least onemetal ion selected from the group consisting of Sr²⁺, Ca²⁺ and Pb³⁺. 5.The method as defined in claim 3 or 4, wherein said preparing stepincludes the step of adding organic acid and polyol to a solution ofmaterial prepared using water-soluble compounds Ba, Zn and Fe as astarting material to form said metal-organic complex constituting saidviscous solution, wherein said viscous solution is formed as a filmhaving a composition of Y-type Ba₂Zn₂Fe₁₂O₂₂ on said surface made ofnoble metal and then burnt.
 6. The method as defined in either one ofclaims 3 to 5, wherein said burning step is performed at a temperatureof 750° C. or more.